Materials corrosion of high temperature alloys immersed in 600C binary nitrate salt.

Abstract

Thirteen high temperature alloys were immersion tested in a 60/40 binary nitrate salt. Samples were interval tested up to 3000 hours at 600°C with air as the ullage gas. Chemical analysis of the molten salt indicated lower nitrite concentrations present in the salt, as predicted by the equilibrium equation. Corrosion rates were generally low for all alloys. Corrosion products were identified using x-ray diffraction and electron microprobe analysis. Fe-Cr based alloys tended to form mixtures of sodium and iron oxides, while Fe-Ni/Cr alloys had similar corrosion products plus oxides of nickel and chromium. Nickel based alloys primarily formed NiO, with chromium oxides near the oxide/base alloy interface. In625 exhibited similar corrosion performance in relation to previous tests, lending confidence in comparisons between past and present experiments. HA230 exhibited internal oxidation that consisted of a nickel/chromium oxide. Alloys with significant aluminum alloying tended to exhibit superior performance, due formation of a thin alumina layer. Soluble corrosion products of chromium, molybdenum, and tungsten were also formed and are thought to be a significant factor in alloy performance.

Corrosion of high temperature alloys in solar salt at 400, 500, and 680ÀC.

“…400 and 500°C samples, which had minimal surface corrosion, were fully cleaned by repeating this chemical bathing process for several cycles. 680°C samples required both chemical baths and mechanical abrasion using grit blasting to complete the removal of corrosion products [4]. Further chemical baths were needed after the grit blasting to remove scales still present on the HA230 and In625.…”

Identified corrosion and erosion mechanisms in SCO2 Brayton Cycles.

“…Molten salts, for use in the Molten Salt Reactor concept, are given as an example to contrast the materials requirements of primary and secondary fluids. Thin chromia layers are soluble in molten salt systems (nitrate, chloride, and fluoride based salts) [3][4][5][6][7][8], making materials selection for heat exchangers a precarious balancing act between high temperature oxidation (S-CO2) and metal dissolution (salt side of heat exchanger).…”

“…This behavior is paralleled in molten nitrate salts, shown by the calculations in Figure 7. Figure 8 shows the correlation between the dissolution of soluble metals and corrosion rates [3].…”

“…The iron cation will react with carbonate to form iron carbonate (FeCO 3 ). An increase in temperature will increase the corrosion rates in CO 2 systems [20].…”

“…Preventive maintenance practices are utilized at SNL to avoid untimely failures [3]. Practices such as periodic bore scope inspection and time-lapsed comparison of components and piping are employed.…”